Contributors: K.R. Barker, Plant Pathologist; D.M. Benson, Research Plant Pathologist; R.K. Jones, Extension Plant Pathologist
The fungi which cause diseases known as powdery mildews attack a wide variety of ornamental plants grown in North Carolina. The disease generally occurs during spring, fall and winter months; during cool weather with high relative humidity; and in shady areas. The damage from infection by the fungi that cause powdery mildew can be slight to severe.
Symptoms. A powdery, fluffy white- to-light-gray-colored fungus growing on succulent stems, leaves, buds and flowers is the characteristic symptom of powdery mildew. Young plants and actively growing shoots are damaged more severely than older plants, leaves or branches. Infected leaves may be dwarfed, curled, or deformed. Powdery mildew can destroy the blossoms of crape myrtle. Symptoms on some varieties of azalea and rhododendron include small, dead flecks in the leaves and defoliation. Infection may also be present without the obvious development of white fungus growth.
Control. Some highly susceptible plants, such as Chinese photinia or euonymus, should be replaced with a similar plant that is not susceptible to powdery mildew. On many trees, the disease causes little or no damage, and control is not necessary. Plants in the landscape which may require fungicide applications to prevent powdery mildew damage include crape myrtle, phlox, rose and zinnia. If damage from powdery mildew is severe and susceptible plants must be grown in the landscape, severely diseased portions must be pruned out, and a labeled fungicide application is recommended. Powdery mildew fungi can become resistant to any one fungicide (except sulfur) when used repeatedly to control the pathogen. The North Carolina Agricultural Chemicals Manual should be consulted for recommended fungicides.
| Scientific Name | Common Name |
|---|---|
| Begonia sp. | Begonia |
| Smoke-tree | |
| Cotoneaster sp. | Cotoneaster |
| Crataegus sp. | Hawthorne |
| Eucalyptus sp. | Eucalyptus |
| Euonymus | |
| Hydrangea snowball | |
| Lagerstroemia indica | Crape myrtle |
| Ligustrum vulgare | Privet |
| Honeysuckle | |
| Magnolia sp. | Magnolia |
| Malus sp. | Apple, Crabapple |
| Phlox | |
| Chinese photinia | |
| Sycamore | |
| Prunus sp. | Peach, plum, apricot |
| Pyracantha sp. | Pyracantha |
| Pyrus sp. | Pear |
| Oak | |
| Rhododendron sp. | Rhododendron, Azalea |
| Rose | |
| Salix sp. | Willow |
| Lilac | |
| Vaccinium sp. | Blueberry |
| Viburnum, Snowball | |
| Wisteria sp. | Wisteria |
| Zinnia | |
Phytophthora root rot is a serious, widespread, and difficult-to-control fungal disease that affects a wide range of plants in North Carolina. Plants susceptible to root rot caused by Phytophthora cinnamomi include azalea, rhododendron, dogwood, Camellia japonica, Pieris, Taxus (yew), deodar cedar, mountain laurel, heather, juniper, high-bush blueberries, Fraser fir, white pine, shortleaf pine, leucothoe, aucuba, and others. Boxwood is attacked by Phytophthora parasitica, a similar fungus.
Symptoms. On azalea, the symptoms vary with the cultivar. On Kurume hybrid types, such as Coral Bells, Hinodegiri, and Hino Crimson, new leaves are smaller than normal with interveinal chlorosis and possibly some purple coloration and defoliation. This chlorosis is often confused with a deficiency of iron or other nutrients. At times, light applications of iron and complete fertilizer does improve the green color of leaves, but only for a short time. Excessive yellowing and loss of older leaves are the predominant symptoms on Snow azalea.
Large plants usually decline slowly in vigor and die branch by branch over a period of several months to years; occasionally they die rapidly. Roots are reddish-brown, brittle and often limited to the upper part of a container or soil. The reddish-brown discoloration advances to the larger roots and eventually to the main stem. On rhododendron, the primary symptoms are rapid wilting and death of leaves. Leaves droop but remain attached to the limb. Watering does not restore leaf turgor. Many shoots usually are affected. Root symptoms are similar to those on azalea.
On Camellia japonica, the main symptoms include a gradual decline in vigor, loss of dark green color, curling of leaves, and excessive loss of older leaves. Root symptoms are similar to those on azalea.
Camellia sasanqua is tolerant to Phytophthora root rot, and this species is often used as a root stock for Camellia japonica.
Taxus dies rather suddenly, with the foliage turning reddish-brown. Roots are also reddish-brown in color and the discoloration may extend into the main stem. White pines in the landscape often die suddenly during dry periods.
Factors Favoring Occurrence. Phytophthora root rot is favored by high soil moisture and warm soil temperatures. The disease does not occur as frequently and may not be as severe on well-drained sandy soils as in heavy clays or poorly drained soils. The disease is common and severe in areas where run-off water and rain water from roofs collect around plant roots. Disease development is also favored by shallow soils with underlying rock or compacted hard pans, setting woody plants deeper than the soil level in the nursery or container, over-watering plants, or long periods of heavy rain.
Prevention/Control. Phytophthora root rot must be prevented, because chemicals often are ineffective in controlling this disease after above-ground symptoms become obvious. The following suggestions may aid in the prevention of root rot:
Long Term Control. For long-term control of root rot, the following control measures should be utilized in this order:
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